Wireguard over VPN broken on windows

[Jason A. Donenfeld]

> 59e556f on wireguard-go breaks

59e556f fixes a regression, which never shipped in any release. There
is nothing here that "once worked and now doesn't." What you have in
mind has never worked.

We're currently using IP_UNICAST_IF on the wireguard socket, attaching
it to the default route. I'd much rather have something like Linux's
policy routing and suppress_prefixlen, but I don't know how to do that
(yet?) on Windows. If you have any ideas or want to do some research,
I'd certainly be very interested.